ES2781224T3 - Valve for closing a fluid line - Google Patents

Abstract

A valve (10) for closing a fluid conduit, comprising: - an inlet element (12) with an inlet conduit (16); - an outlet element (34) with an outlet duct (38); - a closing element (26) that in a first state allows a passage of fluid between the inlet conduit (16) and the outlet conduit (38) and that in a second state closes the passage of fluid, where the closure element closure (26) in the first state is fixedly connected to the input element (12) or the output element (34) and in the second state it is separated from the input element (12) or the output element (34) ; - an actuator (52) which, when actuated, is configured so as to move the closure element (26) from the first state to the second state; and - a pretensioned element (56) that is pretensioned and configured so as to move the closure element (26) to the second state after actuation of the actuator (52); where the actuator (52) and the prestressed element (56) are connected in parallel, characterized in that the actuator (52) extends along a longitudinal axis of the input element (12) or the output element (34) .

Description

DESCRIPTION

Valve for closing a fluid line

The invention relates to a valve for closing a fluid conduit and to a drive system of a space vehicle comprising said valve.

There is evidence that spacecraft and satellites have valves that can be closed and in particular that can be closed only once, that is, irreversibly.

From document DE 10 2014 002 972 A1 there is evidence of a valve comprising a shape memory material which expands when it reaches a phase transition temperature and thereby closes a fluid passage.

From document US 2013/0167377 A1 there is evidence of a valve arrangement for a component of a space vehicle with a single actuation, where the valve arrangement comprises an inlet and an outlet and also an actuator that can be actuated by an element heating. In an actuated state of the actuator, flow between inlet and outlet is possible (valve arrangement closed by default). The actuator comprises a shape memory material that extends in the martensitic state along a longitudinal axis of the actuator and in which the valve arrangement is integrated. When activating the heating element, it undergoes a change in length upon reaching a predetermined transformation temperature, whereby a valve piston can be removed from the sealing seat formed at the inlet or outlet.

US 3974844 A discloses a reversibly operable default open valve arrangement for household appliances having an inlet and an outlet and also an actuator that can be actuated by a heating element. The actuator is a one-way shape memory actuator that is integrated into the valve arrangement and that, through activation of the heating element, undergoes a change in length upon reaching a predetermined transformation temperature, by means of which can gradually press against a sealing seat and remove it from there.

Document US 4973024 A discloses a further arrangement of a closed-by-default valve that can be reversibly actuated, comprising an inlet and an outlet and also an actuator that can be actuated by a heating element.

DE 202012 104460 U1 discloses a reversibly operable default open valve arrangement, having an inlet and an outlet and also an actuator that can be actuated by a heating element and which acts against a return element .

DE 102011 054458 A1 discloses a reversibly operable default open valve arrangement for high voltage battery applications, which has an input and an output and also an actuator that can be actuated by a heating element.

JP 2009 092130 A discloses a further valve arrangement having an inlet and an outlet and also an actuator that can be operated by a heating element.

Document WO 2016/035067 A1 also discloses a valve for closing a fluid conduit according to the preamble of claim 1.

The underlying object of the invention is to provide a valve for closing a fluid conduit, where the valve has a reliable tightness while having a simple construction.

The object is achieved by a valve according to claim 1 and a drive system of a space vehicle according to claim 9.

A valve for closing a fluid conduit has the following: an inlet element with an inlet conduit; an outlet element with an outlet conduit; a closing element, which in a first state allows a passage of fluid between the inlet conduit and the outlet conduit, and which in a second state closes the fluid passage, where the closing element in the first state is connected in a fixed to the input element or the output element and in the second state is removed from the input element or the output element; an actuator which, when actuated, is configured so as to move the closure element from the first state to the second state; and a prestressed element that is pre-tensioned and configured so as to move the closure element to the second state after actuation of the actuator. The prestressed element can be chosen so that it is so strong that the closure element undergoes plastic deformation in the second state, which results in high fluid tightness, in particular high gas tightness. This solution is distinguished by an almost unlimited service life, low production costs and high internal tightness (against helium). The actuator may have a thermal expansion behavior that differs from that of the input element or the output element. In addition, the thermal expansion behavior of the actuator may differ from that of the closing element and / or the input element or the output element, and also from all the others. valve components. Preferably, the actuator expands to a greater extent than the other components. As a result, the degree of thrust against the closure member and the forces transmitted thereby can increase as heating increases, in particular if the closure member prevents expansion of the actuator. In other words, the actuator can be arranged to be designed to exert compressive forces on the closure member in accordance with a heating level.

The thrust of the actuator can take place indirectly, for example by means of intermediate elements, or directly, for example by abutting contact on the closing element. It should be noted that the actuator, in particular at a low heating level or at a low temperature, may in principle not push against the closing element and / or rest directly on it. Instead, this can only occur after a certain heating level or temperature has been reached and, for example, at the latest when a predetermined heating level has been reached, which will be discussed below in the Present. In other words, initially a gap can be provided between the actuator and the closing element, where the gap can in particular compensate for the different thermal expansion behavior of those elements provided that the predetermined heating level has not been reached.

The actuator is further arranged to extend along a longitudinal axis of the input element or the output element. To this end, the actuator may be formed so that it substantially corresponds to the inlet element or the outlet element and / or is arranged within the valve, so that those elements can extend substantially parallel between yes inside the valve. For example, the actuator may extend along a longitudinal axis and may have a generally elongated shape, it being possible for the longitudinal axes of the actuator and the input element or the output element to extend parallel to each other or also they match.

Finally, the actuator can be designed to push against the closure element once a predetermined level of heating has been reached, with a force such that the closure element moves from its starting position.

The predetermined heating level can be understood as, in particular, an increase in temperature of the actuator to a predetermined temperature threshold. Similarly, this may include holding the actuator at or above the corresponding temperature threshold for a specified minimum period of time. The biasing force of the actuator may exceed a force of the weight of the closure element and / or a clamping or pretensioning force that initially holds the closure element in its starting position. In particular, the actuator can be designed so that it moves the closure element substantially along a longitudinal axis of the input element or the output element and / or towards the input element and / or the output element. Additionally or alternatively, the movement may be arranged to take place along the inlet and / or outlet conduit.

Also, the closure element, in its final position moved from the start position, can generally be arranged between the inlet element and the outlet element, thus allowing the fluid to be conducted to flow around or through of this.

The provision of such a valve enables reliable actuation and simplified sealing with a compact construction. In particular, the relevant elements can extend substantially parallel to each other or along common longitudinal axes, and a driving movement can also take place along that longitudinal axis. In general, the valve can therefore have a generally elongated shape and have reduced cross-sectional dimensions compared to already known solutions. In particular, the dimensions of the valve extending transverse to a fluid conduit through the valve can be considerably reduced.

The actuator may comprise a hollow cylindrical part that surrounds at least a part of the input element or the output element. Surrounding at least one part can be arranged along the longitudinal axis of the input element or the output element and / or along a peripheral axis. However, the actuator preferably extends at least along a certain part of the longitudinal axis of the input element or the output element and thus receives it in the hollow cylindrical part. At least the hollow cylindrical part of the actuator can be formed generally corresponding to the input element or the output element, for example with a corresponding cross-sectional shape. On the other hand, the hollow cylindrical part can rest on an outer peripheral surface of the input element or the output element and / or at least a part of this can be arranged at a certain distance from these, so that there is a gap between a inner peripheral surface of the actuator and an outer peripheral surface of the input element or the output element. At a higher level, the hollow cylindrical part may consequently be tubular or cap-shaped, preferably with a rounded and / or circular cross section, and the actuator may have one as if it had to be pushed onto the input element. or the output element.

According to a further aspect, the actuator, at the latest when the predetermined heating level is reached, can be moved at least in part relative to the input element or the output element, and in particular, it can be moved along along the longitudinal axis of the input element or the output element. In In particular, the relative displacement capacity can be attributed to an increasing thermal expansion of the actuator relative to the input or output element, which remains constant or expands only slightly. In particular, it can be arranged that the inlet element and / or the outlet element is (are) generally stationary inside the valve and that the actuator moves relative to them according to the level of pressure. heating. The actuator can be elongated in general, so that in particular an axial end of the actuator facing the exit region can be moved relative to and / or along the inlet element or the outlet element.

It can further be arranged that the actuator, at the latest when the predetermined heating level is reached, has at least one dimension that increases compared to a state of the actuator below the predetermined heating level. For example, the actuator can be lengthened and / or its cross-sectional dimensions changed depending on the level of heating. In particular, this can be an elongation along a longitudinal axis of the actuator and / or the input element and / or the output element.

According to a preferred variant, the actuator comprises a shape memory material. Such materials are known in the prior art, for example in the form of so-called shape memory alloys (in particular nickel titanium alloys). Shape memory material can also be activated by heat or temperature. In other words, the shape memory material, according to a heating level of the actuator, can revert to an original starting shape which is preferably distinguished as indicated above by larger dimensions and / or higher stress forces. push against the coupling element.

In this connection, the shape memory material can be pseudoplastically deformed and begin to return to its original shape, at the latest when the predetermined heating level is reached. Pseudoplastic deformation can include compression of the actuator and preferably occurs with the application of compressive forces on the actuator.

Consequently, the actuator can be initially arranged in a deformed state (pseudoplastically) in the valve, it being possible for the actuator in this state to also have reduced dimensions (for example, a reduced length due to compressive compression forces) . In this way, the twinning of the microstructure of the material, of the shape memory material, can be undone in a known manner in order to provide the shape memory effect. The pseudoplastic deformation can also persist initially in the absence of heat or with only slight heating. According to a heating level of the actuator, and in particular when an activation temperature or an activation heating level is reached, the pseudoplastic deformation can be reversed in this case, so that the actuator returns to its original shape. As indicated, this may in particular include a return to an uncompressed state, that is, for example, a corresponding increase and / or elongation of the actuator. This can generally result in greater thermal expansion of the actuator compared to the input element, as noted above, and leads to higher biasing forces against the closure element.

In its starting position, the closure element may be connected to the inlet element or the outlet element by means of a material bonding connection. In this case too, this arrangement, and in particular a materials bonding connection, can be provided directly with an axial end of the inlet element or the outlet element, preferably oriented towards the inlet element or the outlet element. The materials bonding connection can be provided by welding, adhesive bonding, brazing, one-piece forming or the like, whereby the materials bonding connection can be generally encompassed by a connection portion between the element. input or output element and closing element.

It can further be arranged in this connection that a connecting part between the inlet member or the outlet member and the closure member includes a mechanically weakened region, which forms a predetermined breaking point. Mechanical weakening can be understood to mean in particular a lower force transmission capacity, or a lower breaking point, compared to adjacent regions of the input element or of the output element or of the connection part. This can generally be done by modifying the properties of the material in the region of the predetermined break point, for example by choosing different materials within the input or output element. Similarly, the stiffness in the region of the predetermined breaking point can be reduced, for example, by modifying the dimensions of the cross section. According to a preferred variant, the predetermined breaking point comprises a notch in the form of a smaller local cross section, and in particular, in the form of a smaller local diameter in the case of a cylindrical (and / or tubular) shape of the element. input or output. Accordingly, the closure element may be in the form of a removable notched tubular head. Accordingly, the actuator can be arranged to be designed so that it pushes against the closure element, when the predetermined heating level is reached, with such a force that the predetermined breaking point is broken and thus a movement of the closing element from its starting position.

The closure element may comprise at least one force transmission region against which the actuator can push, at the latest when the predetermined heating level has been reached. The force transmission region can comprise, for example, a part of the closure element having a larger cross section, and in particular, the dimensions of the diameter, which preferably protrudes with respect to a diameter of the inlet or outlet element. The force transmission region can further extend opposite an axial end or face of the axial end of the actuator (in particular, when viewed along the longitudinal axis), so that the actuator, when heated, can expand towards the force transmission region and / or resting on it with increasing thrust forces.

The prestressed member can push against the closure member and can be designed to move the closure member from its starting position / first state at the latest when the predetermined heating level is reached. The prestressed element can be accommodated in the valve with suitable prestressing and pushed against the closure element, in particular by means of a compressive force. In a preferred variant, the prestressing device similarly extends along a longitudinal axis of the input or output element. Furthermore, it can thereby be arranged in a gap between the actuator and the input element and / or similarly pushed against a possible force transmission region of the closure element. Finally, the prestressed element may take the general form of a spiral spring, preferably a helical spring, and receive at least a part of the input or output element or extend in the form of a spiral around the input or output element.

The provision of a prestressed element is particularly suitable for variants in which a predetermined breaking point must first be broken by means of the actuator and the prestressed element must then ensure sufficient movement of the closure element from its starting position (eg a complete withdrawal from an axial end of the input element). To this end, the prestressed element can further be designed so as to move the closure element far enough that it is withdrawn from the actuator, which may be lengthening. The prestressed member can be designed to move the closure member (eg, in the form of a separate notched tubular head) to a predetermined stop. The stopper can be arranged on a connecting element, on a wall of the connecting element or on the inlet or outlet element as a sealing seat. Likewise, the closure element, when it is displaced by the prestressed element, can push against the connection element and in particular against an inner wall of the connection element or can slide directly on it.

The actuator and the prestressed element are connected in parallel. As a result, a compact space-saving valve can be provided, and the force to close the valve can be increased compared to a series connection.

Alternatively or additionally, the prestressed element can be arranged, at least partially, inside the actuator or the actuator can be arranged, at least partially, inside the prestressed element, which not only makes the valve more compact, but which also affects the mutual guidance of the prestressed element and the actuator, so that their development of force is directed.

The inlet element may have at least one radial hole connecting the inlet duct with an outer part of the inlet element, the radial hole (s) preferably having a smaller diameter than the inlet duct. Such a radial hole can improve the compactness of the valve.

The radial hole (s) may lead to a fluid passage chamber connecting the inlet conduit and the outlet conduit so as to conduct the fluid.

Similarly, a connecting element can connect the inlet element and the outlet element so as to conduct the fluid and defines together with the inlet element and the outlet element the fluid passage chamber. In this way, the assembly of the valve components, in particular the prestressed element and / or the actuator, can be facilitated and a material can be chosen which is different from the material of the inlet and outlet element and / or a thickness of the material that is different from the material thickness of the inlet and outlet element for the connecting element. Said connecting element preferably has different heat conduction properties than the input or output element, preferably better heat conduction properties. As a result, the actuator can be actuated in advance or actuated prior to thermal expansion of the input or output element. Preferably, the connecting element is made of a NiCr or Ti alloy.

The connecting element may be substantially cylindrical and preferably have an elongated shape, and it may also be in one piece or composed of a plurality of individual components. In particular, the connecting element can be tubular or cap-shaped and can have a coupling region for the inlet element and also for the outlet element. The connecting element can further extend along a longitudinal axis, which can extend parallel to and preferably coincide with the longitudinal axes of the input element and / or of the actuator.

In a variant, at least the input element is welded to the connecting element. Additionally or alternatively, any outlet element can also be welded to the connecting element. The actuator and also the connecting element can thereby be arranged inside the connecting element and thus enclosed in a fluid-sealed manner. The welded connections can further ensure a sufficient sealing of the valve, and the inlet and outlet element and preferably also the connection element can be hollow cylindrical or substantially tubular. Due to this welding a particularly tight valve (external tightness) is provided.

In principle it is conceivable that the heating of the actuator takes place without the supply of intentional or artificially generated heat, but that it occurs automatically as a result of a general heating of the drive system after re-entry into the atmosphere. For example, there is evidence that considerable frictional heat is generated, which generally heats a satellite, for example, and its components, upon re-entry into the atmosphere due to aerothermodynamic effects.

Furthermore, a heating element may be thermally connected to the actuator, which comprises a shape memory material, the input element and / or the output element being thermally decoupled from the heating element by means of a thermal decoupling element, for example. means of which a short release time / valve closing time can be achieved. The heating element may preferably be adhesively bonded to the connecting element. The decoupling element can be made of a silicone, an elastomer, or a ceramic oxide material, as well as a combination of these materials.

Similarly, the closure element can be a body that extends in the longitudinal direction of the valve and is guided predominantly within the prestressed element. Thus, the mass of the closure member can be increased and the closure member can serve as a guide for the prestressed member and / or the actuator.

It is also possible for the closure element in the second state to be pressed into a sealing seat, the closure element and / or the sealing seat being provided with a soft metallic coating, preferably a galvanic gold deposition, by means of which can reduce the forces required to achieve permanent sealing. The above-described greater mass of the closure element due to its extension direction in the longitudinal direction of the valve leads to a higher mass moment of inertia, which results in a higher plastic deformation of the closure element in the sealing seat.

The invention further relates to a drive system for a space vehicle, in particular a drive system for a satellite, comprising one of the above valves.

The preferred embodiments of the invention are explained in more detail below by means of the attached drawing, in which

Figure 1 is a longitudinal section of a valve, according to an exemplary embodiment, which is in an open start state.

Figure 1 shows a valve according to an exemplary embodiment, where the valve is designated generally 10.

The construction of the valve 10 will be described first, followed by its mode of operation.

Valve 10 is generally cylindrical in shape and extends symmetrically along a longitudinal axis L, which coincides with the longitudinal axes of all subsequent valve components.

Valve 10 has a tubular inlet member 12 that can be coupled with a fluid conduit (not shown), for example, of a spacecraft drive system, in particular a satellite drive system. Starting from an end 14 suitable for engaging a fluid conduit, inlet member 12 extends tubularly along longitudinal axis L, with inlet conduit 16 in the form of a constant diameter hole 16. Along this hole 16, the cylindrical inlet element 12 has, in the longitudinal direction, a mounting portion 18 in the form of a cylindrical projection, which will be explained in more detail hereinafter and, following this mounting portion 18, through holes 20 extending radially through inlet element 12, connecting orifice 16, that is, the interior of inlet element 12, with the exterior of inlet element 12 in a fluid-permeable manner.

The through holes 20 herein extend equidistant along a plane through the inlet member 12 and through the longitudinal axis L, in each case two groups of four of said through holes 20 are arranged perpendicular to each other. Furthermore, the through holes 20 are inclined in their direction of extension from the hole 16 outwards in the opposite direction to the end 14 of the inlet element 12. Following the through holes 20, the hole 16 ends as a blind hole 22 of smaller diameter. .

In an intermediate region of the blind hole 22 along the longitudinal axis L, the wall thickness of the inlet element 12 is reduced to a minimum along the inlet element 12 by means of a notch 24 running radially in the exterior of the input element 12. This notch 24 constitutes a predetermined breaking point. The blind hole 22 is followed by a closure element 26 in the form of a solid cylinder, which extends in the form of a cylinder for two thirds of its length along the longitudinal axis L and in its final third it widens radially so annular to a conduit portion 28 with the through holes 30 extending parallel to the axis longitudinal L, in order subsequently, starting from the same outer diameter as the first two-thirds of the closing element 26, to end up with a dome-shaped sealing piston 32.

Additionally, valve 10 has an outlet element 34 which, like inlet element 12, can be coupled with a fluid conduit (not shown), for example, of a spacecraft drive system, in particular a satellite drive system. Starting from an end 36 suitable for engagement with a fluid conduit, outlet member 34 extends tubularly along longitudinal axis L with outlet conduit 38 in the form of a hole 38 of constant diameter. The outlet element 34 herein is in the form of a hollow cylinder with multiple steps, a first cylindrical projection 40 spaced from the end 36 that combines with a second projection 41. Within the second projection 41, the hole 38 widens into the end of the outlet element 34 which faces the closure element 26 to form a sealing seat 42 in the shape of a cone.

The sealing piston 32 and / or the sealing seat 42 are provided with a soft metallic coating, preferably a galvanic gold deposition.

A connecting element 44 of a NiCr or Ti alloy in the form of a valve housing 44 connects the inlet element 12 and the outlet element 34 by means of, in each case, a peripheral weld bead 46, in such a way that the sealing piston 32 and the sealing seat 42 are separated from each other along the longitudinal axis L by a distance S in an initial non-actuated state of the valve. The connecting element 44 is a stepped hollow cylinder, where a first region 48 of smaller inside diameter can be pushed over the mounting portion 18 of the input element 12 and a second region 50 with a larger inside diameter can be pushed over the second projection 41 of the outlet element 41 so that it is in abutment and flush with it. The second region 50 has an inner diameter greater than the outer diameter of the inlet element 12 at the through-holes 20, so that a fluid passage chamber 51 is present inside the connecting element 44 between the element. input 12 and output element 34.

An actuator 52 extends in the form of a hollow cylinder along the inner wall of the connecting element 44, between the first region 48 and the conduit portion 28. At its end 54 facing the conduit portion 28, the actuator 52 is radially inwardly staggered in the shape of a cylinder at a distance from closure element 26. Actuator 52 is a shape memory actuator 52 that is compressed prior to fitting into connecting element 44 so that it does not fit. expand until it reaches a phase transition temperature.

A pre-tensioned prestressed element in the form of a coil spring 56 extends along the inner wall of actuator 52, between first region 48 and stepped end 54 of actuator 52.

An electrical heating element 58 is adhesively bonded to the outer side surface of connection element 44. Heating element 58 extends along longitudinal axis L in a hollow cylindrical manner concentrically with actuator 52, so that it surrounds a large proportion of the actuator 52. The shortest distance to the heating element 58 from the input element 12 and the output element 34 is thus always greater than the shortest distance to the heating element 58 from the actuator 52.

The thermal decoupling elements 60 are located on the outer side surface of the first region 48 of the connecting element 44 and on the outer lateral surface of the first projection 40 of the outlet element 34. The decoupling elements 60 can be made of silicone , an elastomer or a ceramic oxide material, as well as a combination of these materials.

The mode of operation of valve 10 will now be described.

In the first state of valve 10 shown in FIG. 1 a fluid conduit is open. Arrows show fluid flow through inlet conduit 16, through holes 20 in inlet element 12, fluid passage 51, inside stepped end 54 of actuator 52, through holes 30 in conduit portion 28 and outlet conduit 38.

In a second state (not shown) of valve 10, actuator 52 has expanded after reaching a phase transition temperature due to the action of heat from heating element 58 and, together with prestressed element 56, has a limit of the breaking force has been reached, which leads to the closure element 26 being dislodged from the input element 12 in the notch 24. The prestressing element 56 then presses the detached closure element 26 with its sealing piston 32 towards the sealing seat 42, in which the prestressed element 56 also continues to reliably retain the sealing piston 32. In this second state, the fluid conduit is thus separated in a fluid-tight manner at the point between the through holes 30 of the conduit portion 28 and the outlet conduit 38. As a result of the breaking operation, the closure element 26 moves away from the inlet element 12, this state is irreversible, that is, the valve 10 can only be closed once.

Claims (9)

1. A valve (10) for closing a fluid conduit, comprising: - an inlet element (12) with an inlet duct (16); - an outlet element (34) with an outlet conduit (38); - a closing element (26) that in a first state allows a passage of fluid between the inlet conduit (16) and the outlet conduit (38) and that in a second state closes the fluid passage, where the closure (26) in the first state is fixedly connected to the input element (12) or the output element (34) and in the second state it is separated from the input element (12) or the output element (34) ; - an actuator (52) which, when actuated, is configured so as to move the closure element (26) from the first state to the second state; and - a prestressed element (56) which is pre-tensioned and configured so as to move the closure element (26) to the second state after actuation of the actuator (52); where the actuator (52) and the prestressed element (56) are connected in parallel, characterized in that The actuator (52) extends along a longitudinal axis of the input element (12) or the output element (34). 2. The valve (10) as claimed in claim 1, where the prestressed element (56) is arranged, at least partially, inside the actuator (52) or the actuator (52) is arranged, at least partially, inside the prestressed element (56). The valve (10) as claimed in any one of the preceding claims, wherein the inlet element (12) has at least one radial hole (20) connecting the inlet conduit (16) with an outer part of the inlet element. input (12). 4. The valve (10) as claimed in claim 3, where the radial hole (s) (20) lead to a fluid passage chamber (51) that connects the inlet conduit (16) and the outlet conduit (38) in a way that conducts the fluid. The valve (10) as claimed in claim 4, where a connecting element (44) connects the inlet element (12) and the outlet element (34) in a way that conducts fluid and together with the inlet element (12) and the outlet element (34) defines the fluid passage chamber (51). 6. The valve (10) as claimed in any of the preceding claims, where a heating element (58) is thermally connected to the actuator (52), comprising a shape memory material and at least the input element (12) and / or the output element (34) is (are) decoupled (s) thermally from the heating element (58) by means of a thermal decoupling element (60). The valve (10) as claimed in any one of the preceding claims, where the closure element (26) is a cylindrical body that extends in the longitudinal direction (L) of the valve (10) and is guided predominantly inside the prestressed element (56). The valve (10) as claimed in any one of the preceding claims, wherein the closure element (26) in the second state presses on a sealing seat (42), at least one of the closure element (26) and the sealing seat (42) being provided with a soft metallic coating. 9. A space vehicle drive system, in particular a satellite drive system, comprising a valve (10) as claimed in any one of the preceding claims.